logical operators

&& 

    and (logical)

    if [ $condition1 ] && [ $condition2 ]
    # Same as:  if [ $condition1 -a $condition2 ]
    # Returns true if both condition1 and condition2 hold true...

    if [[ $condition1 && $condition2 ]]    # Also works.

|| 

    or (logical)

    if [ $condition1 ] || [ $condition2 ]
    # Same as:  if [ $condition1 -o $condition2 ]
    # Returns true if either condition1 or condition2 holds true...

    if [[ $condition1 || $condition2 ]]    # Also works.
    # Note that || operator not permitted within [ ... ] construct.

Example:

    X=1
    [ $X -ne 0 ] && echo "X isn't zero" || echo "X is zero"

reference

• Shell Basic Operators
• Advanced Bash-Scripting Guide: Operations